This invention relates generally to a water heater. More particularly, it relates to a quick recovery water heater which incorporates an external heat source, for example a heat pump, and which is adapted for use in a domestic water supply system.
The typical domestic electric water heater includes a water storage tank having a tank heating unit provided with upper and lower heating devices, generally electric resistance heating elements, respectively associated with upper and lower tank thermostats. The arrangement is such that water in the upper portion of the tank is heated until the upper tank thermostat is satisfied, and then water in the lower portion of the tank is heated until the lower tank thermostat is satisfied. As a result, a limited supply of hot water is replenished in a relatively short period of time, to the extent of the capacity of the upper portion of the tank.
The heat pump has been known for many years. It is generally more economical to operate than a conventional tank heating unit. However, the heat pump has not been used extensively heretofore because power, particularly electric power, has been inexpensive. As the cost of power increases, there is renewed interest in the use of a heat pump in association with a water heater, especially for providing domestic hot water.
When a heat pump is used in association with a water heater, it is advantageous to make the heat pump responsive to the tank thermostats as an indicator of the demand for hot water. Arrangements of this nature are disclosed in U.S. Pat. No. 4,314,456 issued Feb. 9, 1982, U.S. application Ser. No. 416,435 filed Sept. 10, 1982 and U.S. application Ser. No. 450,499 filed Dec. 16, 1982. All are of common assignee herewith and are incorporated herein by reference.
The U.S. Pat. No. 4,314,456 requires both a thermostatically operated valve and a by-pass valve. Water leaving the heat pump is maintained at an elevated temperature until the demand in the upper portion of the tank has been satisfied. An inherent disadvantage of this is that the heat pump is least efficient when operating at an elevated temperature.
Neither of the two U.S. application Ser. Nos. 416,435 or 450,499 provides for rapid replenishment of hot water in the upper portion of the tank. Rather, each uses a heat pump to heat an entire tank of water in a single batch. This arrangement necessarily does not provide for quick recovery; that is, it does not provide for rapid replenishment of the supply of water stored in the upper portion of the tank. Operation in this manner also is inefficient.
There remains a need in the art for a quick recovery heat pump water heater which will initially heat water in the upper portion of a water storage tank, and subsequently heat water in the lower portion of the tank. It should operate efficiently, and should be responsive to the tank thermostats as an indicator of the demand for hot water. It should be conducive to installation on-site in the field as an adjunct to an existing conventional domestic water heater.